224 ANNUAL RECORD OE SCIENCE AND INDUSTRY. 

 CONSTITUTION OF AMMONIUM AND ITS DERIVATIVES. 



Valuable to the science of chemistry as the theory of 

 equivalence has been, it has yet very much to do before it 

 can be admitted to be complete. Indeed, the signs of the 

 times point to a period not very distant when it will be merged 

 into some higher and broader generalization. Chemists are 

 not now agreed, for example, upon a point which would 

 seem to lie at the very foundation of such a theory: namely, 

 whether the equivalence of an element is a fixed quantity for 

 that element, or whether it can vary. In the case of nitro- 

 gen, for example, all are agreed that it may and does act as 

 a triad in ammonia ; but in ammonium chloride, NH 4 C1, 

 where it is combined with five monad atoms, all are not will- 

 ing to concede that it is quinquivalent. Meyer and Lecco 

 have sought to throw some light upon this question by a 

 careful study of the compound ammoniums. If two of the 

 four atoms of hydrogen in ammonium chloride be replaced by 

 ethyl and two by methyl, two isomeric bodies can be form- 

 ed if the nitrogen be a triad thus N(CH 3 ) 2 C 2 H 5 +C 2 H 5 C1 

 and N(C 2 IT 5 ) 2 CH3+CH 3 C1 ; while, if it be a pentad, these 

 bodies, prepared even though they be in different ways, 

 must be identical. The former of the two was prepared from 

 dimethylamine, and the latter from diethylamine; and the 

 compounds themselves as well as their derivatives were sub- 

 jected to the most careful scrutiny, but not the smallest dif- 

 ference could be observed between them. A critic having 

 suggested that possibly a rearrangement of atoms within the 

 molecule having taken place caused this similarity, the au- 

 thors specially tested the matter, but with a negative result. 

 They maintain, therefore, that the equivalence of nitrogen in 

 ammonium is five, and that equivalence is variable. 35 C, 

 VIII., 233, March, 1875. 



WHY DOES PLASTER OF PARIS SET? 



Landrin has examined the chemical and physical changes 

 which are produced in the setting of plaster of Paris. He 

 notices that three separate actions take place, and that these 

 may very readily be observed under the microscope. They 

 are : First, the burned plaster in contact with the water as- 

 sumes a crystalline form. Second, the water which envel- 



